An acrylic fiber is generally used for pile fabric such as boa and high-pile, because of the touch feeling and appearance pro vides animal-hair like texture and gloss and have been extensively used as a pile material for an animal-hair like pile fabrics cloth such as a boa and a high pile cloth. Among such pile fabrics, for example, pile portion of a high pile cloth generally has a double layer structure composed of guard hair and down hair. The double layer structure can be provided as follows. At first, raw staple fibers of guard hair and down hair are mixed to form a sliver, which is then knitted with ground yarn using the knitting machine to be knit structure. Then, a resin is applied to the back-face of the knitting fabric, and is cured by a heating apparatus known as a tenter. The shrinkage fiber for down hair shrinks during this curing process. Finally, the fiber for guard hair is extended by removing crimp in a polishing process. Therefore, a high shrinkage acrylic fiber is suitable for a staple fiber for down hair. Furthermore, it is necessary that a shrinkage fiber for down hair have a high shrinkage potential under the dry heat condition, because the curing is dry heating process.
There have been proposed a number of methods for preparing a high shrinkage acrylic fiber. For example, coagulated undrawn filaments obtained by wet spinning is drawn to 1.5 to 3 folds, then washed with water at a temperature of 70° C. or higher without tension, and are then subjected to secondary drawing to 1.5 to 2.5 folds, to provide such a fiber (e.g., see Japanese Patent Laid-open Publication No. 110910/1985; Patent Reference 1). Shrinkage level of the acrylic fiber is good in boiling water, but it is poor under a dry heating condition. There has been proposed a quick-shrinking acrylic fiber made of a polymer comprising 80% or more by weight of acrylonitrile and 5 to 15% by weight of a sulfonic-containing monomer and having a dry-heating shrinkage rate of no less than 30% under 10 min at 120° C. (e.g., see Japanese Patent Laid-open Publication No. 119114/1992; Patent Reference 2). Furthermore, there has been proposed a shrinkable staple fiber having 1 to 3 secondary crimps/inch with a shrinkage rate of no less than 15%, irrespective of dry or wet heating (e.g., see Japanese Patent Laid-open Publication No. 316750/1997; Patent Reference 3).
However, in a manufacturing process for a high pile fabric, there is two heating process, curing at the tenter and polishing. Usually the polishing temperature is higher than the curing one. For a conventional high shrinkage acrylic fiber described above, excessive shrinkage may occur during the polishing process. Consequently, it is difficult to provide a product having satisfactory properties in terms of softness and bulkiness. A process temperature in a tenter often varies depending on a processing rate and the type of the tenter. There are many product specifications for a high pile fabric depending on a wide variety of needs in the market. Therefore, it isn't enough that a fiber for down hair simply has a high shrinkage potential, because shrinkage level is to be unevenness depending on temperature conditions in the tenter. As the results, product properties have not been met various requirements to individual high pile fabric.
On the other hand, a staple fiber for guard hair gives appearance and touch feeling like a natural fur to high pile fabric by removing crimp. Thus, a staple fiber for guard hair is preferably a fiber whose crimp can be easily removed by polishing. However, since a conventional acrylic fiber is generally dyed after crimping, it is difficult to remove crimp, which has been exposed to heat history at an elevated temperature during a dyeing process. Such a problem of difficulty in ruffle removing is generally solved by increasing the number of polishing in the polishing process or conducting polishing at a higher temperature. Such increase in a polishing number or temperature may cause ruffle removal in a down hair fiber in which ruffle removal is not necessary, often resulting in a product without bulkiness or high-quality texture.
<List of Prior Art>
    Patent Reference 1: Japanese Patent Laid-open Publication No. 110910/1985    Patent Reference 2: Japanese Patent Laid-open Publication No. 119114/1992    Patent Reference 3: Japanese Patent Laid-open Publication No. 316750/1997.